Method and device for chronologically synchronizing a location network

ABSTRACT

A positioning system that includes a transceiver (hereafter termed a Positioning-Unit Device) that receives one or more reference positioning signals from other Positioning-Unit Devices and/or from other qualified reference positioning signal sources. Each of the received reference positioning signals preferably has a carrier component, a pseudo-random code component, and a data component. The Positioning-Unit Device generates, in response to the received reference positioning signals and their known locations, a unique positioning signal. The unique positioning signal has a carrier component chronologically synchronized to one or more of the carrier components of the received reference positioning signals, a pseudo-random code component chronologically synchronized to one or more of the pseudo-random code components of the received reference positioning signals, and a data component chronologically synchronized to one or more of the data components of the received reference positioning signals. Once a Positioning-Unit Device is chronologically synchronized to a reference transmitter other Positioning-Unit Devices entering the network can use its transmitted unique positioning signal as a reference positioning signal. The geographical distribution of these chronologically synchronized Positioning-Unit Devices creates a time-coherent network of positioning signals that propagate a reference timebase over a substantial geographical area. The positioning system also includes a roving position receiver. The roving position receiver can generate code-based single point position determinations by making range measurements for each of the received chronologically synchronized pseudorandom code and data components, and can generate carrier-based single point position determinations by making range measurements for each of the received chronologically synchronized carrier components. The formation of a chronologically synchronized positioning network allows a roving position receiver the ability to autonomously calculate both code and carrier-based single point position solutions without the requirement for differential correction or absolute time accuracy within the network.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods forgenerating precise position determinations for a mobile apparatus. Inparticular, the present invention applies to precise time-of-arrivalposition determination systems. The present invention is not constrainedby prior art requirements such as physical connections betweentransmitter beacons, such as the need for atomic time standardsconnected to each transmitter, or the need for differential correctiontechniques.

BACKGROUND OF THE INVENTION

It is well understood in the art that precise time-of-arrival positiondetermination is dependant upon the accuracy of the transmitter clocksused. In its most rudimentary form, three transmitter beacons positionedat known locations and connected to a common clock via three identicallength cables will suffice as the basis for a time-of-arrivalpositioning system. However this rudimentary positioning system ishighly impractical to manufacture and install due to the requirement forprecisely timed cables distributing high frequency timing signals overpotentially large distances between beacons. Alternatively, precisionatomic time standards, which have very low drift rates, may be installedat each transmitter beacon and monitored using a reference receiverpositioned at a known location and connected to a reference timebase. Inresponse to positioning signals received from the transmitter beacons,clock corrections are sent from the reference receiver via an RF datalink to each beacon, for subsequent retransmission to user equipment.Modern satellite positioning technologies such as GPS employ thistechnique, wherein cesium and rubidium time standards are installed ineach GPS satellite, with the GPS Ground Control Segment continuallymonitoring all GPS satellites and up-linking clock corrections to eachsatellite every twenty four hours. These corrections are thenrebroadcast via each satellite's navigation message to OPS userequipment, so that positioning algorithms within the GPS user equipmentcan account for satellite clock error. With at least four GPS satellitesin view, a three-dimensional position is accomplished in GPS userequipment using a standard technique known as a conventional code-basedGPS position solution. This standard technique is also generally termed“a single point position” by those skilled in the art.

Conventional Code-Based GPS Position Solution (Single Point Position)

In conventional code-based GPS, the latitude, longitude, and altitude ofany point close to the earth can be calculated from the propagationtimes of the positioning signals from at least four GPS satellites inview. A GPS receiver makes range computations based on the correlationof internally generated pseudorandom code (PRN) sequences with receivedpseudorandom code sequences from each GPS satellite. The measured rangesare referred to as pseudoranges as there is a time difference, oroffset, between the clocks on the satellites and the clock within theGPS receiver. It is necessary to ensure that the receiver's clock issynchronized with the satellite constellation's clock in order toaccurately measure the elapsed time between a satellite's pseudorandomcode sequence transmission and reception of that pseudorandom codesequence by a GPS receiver. A navigation message is also transmittedfrom each satellite, which includes time information, satellite orbitalinformation, and satellite clock correction terms. For three-dimensionalpositioning a GPS receiver requires four satellite signals to solve forthe four unknowns of position (x, y, z) and time (t). Fortwo-dimensional (2-D) positioning, altitude is fixed, and threesatellite signals are required to solve for three unknowns of position(x and y) and time (t). A conventional code-based GPS position solutionis able to provide a GPS receiver, with at least four satellites inview, the capability to determine an absolute three-dimensional (3-D)position with an accuracy of approximately 10 to 20 meters.

This Conventional Code-based GPS position solution is an autonomoussolution, which can determine position, velocity, and time (PVT) withoutdifferential correction data from reference receivers. It has thereforebecome known as a “single point” position solution in the art.

Conventional Code-Based Differential GPS (Relative Positioning)

With an established accurate atomic timebase the GPS constellation isonly capable of providing a GPS receiver with an absolutethree-dimensional position accuracy of approximately 10 to 20 meters.This is due to the corruption of positioning signals from six majorerror sources: (1) ionospheric delay, (2) tropospheric delay, (3)ephemeris error, (4) satellite clock error, (5) GPS receiver noise and,(6) multipath. Ionospheric delay is the varying time delay experiencedby electromagnetic waves when passing through bands of ionized particlesin the ionosphere. Tropospheric delay is the time delay experienced byelectromagnetic waves when passing through moisture in the loweratmosphere. Ephemeris error is the difference between the actualsatellite location and the position predicted by satellite orbital data.Receiver noise is the noise generated by the internal electronics of aOPS receiver. Multipath is the signal delay caused by localized signalreflections in close proximity to a GPS receiver. The majority of theseerror sources are spatially correlated over relatively short distances(i.e. tens of kilometers). This means that two different GPS receiverswithin this proximity to one another will observe the same errors. It istherefore possible to improve the spatially correlated error sourcesusing a technique known as “Differential Correction”. A referencereceiver placed at a well-known location computes an assumed pseudorangefor each satellite signal it detects. It then measures the receivedpseudoranges from the GPS satellites and subtracts the assumedpseudoranges from the received pseudoranges, forming a differentialrange correction for each satellite in view. The reference receiver thensends these corrections as digital data to the GPS receiver via an RFdata link. The GPS receiver subsequently adds these corrections to thepseudoranges it measures (for the same satellites in view to thereference receiver) before calculating a position solution. Errorscommon to both reference receiver and the GPS receiver are completelyremoved by this procedure. Uncorrelated error sources such as multipathand receiver noise remain in the pseudoranges and subsequently degradeposition accuracy. Position accuracies in the order of several metersare achievable with code-based differential GPS correction in lowmultipath environments.

Conventional Carrier-Based Differential GPS (Relative Positioning)

Conventional carrier-based differential GPS (CDGPS) calculates thedifference between the reference location and the user location usingthe differences between the carrier phases of the satellites measured atthe reference receiver and the user receiver. A CDGPS referencereceiver, installed at a well-known location, calculates simultaneouscarrier phase measurements for all satellites in view, and thenbroadcasts carrier phase data to the user receiver via an RF data link.The user receiver also calculates simultaneous phase measurements forall satellites in view, and subsequently computes a phase difference todetermine the position of the user receiver with respect to thereference receiver location. The carrier phase measurements are arunning cycle count based on the Doppler frequency shift present on thecarrier frequencies from the GPS satellites. Each epoch, this runningcycle count (the value from the previous epoch plus the advance in phaseduring the present epoch) is available from the receiver. Morespecifically, the advance in carrier phase during an epoch is determinedby integrating the carrier Doppler offset over the interval of theepoch, hence the name Integrated Carrier Phase (ICP).

The user receiver can measure the fractional phase plus an arbitrarynumber of whole cycles of the carrier, but cannot directly determine theexact number of whole cycles in the pseudorange. This number, known asthe “integer cycle ambiguity”, must be determined by other means.Traditional strategies for resolving carrier phase integer ambiguitiesfall into three broad classes: search methods, filtering methods, andgeometrical methods. These traditional methods do not yieldinstantaneous integer cycle ambiguity resolution. A technique, known as“wide-laning”, has been developed to overcome the non-instantaneousinteger cycle ambiguity problem. Wide-laning multiplies and filters twocarrier frequencies (traditionally the GPS L1 and L2 frequencies) toform a beat frequency signal. This beat frequency wavelength issignificantly longer than the wavelengths of the two individualcarriers. Consequently, resolution of the integers can be accomplishedby using pseudorange observations to determine the integer ambiguity ofthe wider “lanes” formed by the beat frequency signal. These, in turn,greatly reduce the volume of integers that must be searched to resolvethe integer ambiguity.

The main constraints for CDGPS methods are firstly the integrity andlatency of the RF data link, and, secondly, the lack of timedetermination at the user receiver. The data bandwidth of the RF datalink constrains differential data update rates, causing data latency anddegrading position accuracy. Poor reception of differential data causedby physical obstruction and multipath causes data corruption, whichdegrades position accuracy at best, and results in total link failureand no position update at worst. The second shortcoming of CDGPS is thelack of time determination. A conventional single point positionsolution solves for the four unknowns of position (x, y, z) and time(t). CDGPS uses a process known as “double differences”, whicheliminates the receiver clock terms for both the reference receiver andthe user receiver. Therefore, the user receiver can determine accurateposition with respect to the reference receiver position, but cannotdetermine time. This is unimportant if the user is simply, and only,interested in position. However, precise knowledge of an accurate systemtimebase is very beneficial to many user applications involving computernetworks and telecommunication systems. The lack of time determinationis a major problem associated with CDGPS prior art systems.

Pseudolite Augmentation

Another approach used to aid GPS position determination is the use ofground-based augmentation systems such as pseudolites. Pseudolites canbe incorporated into Conventional Code and Carrier-based DifferentialGPS systems without any additional infrastructure requirements. They canbe used as additional ranging signals, and also as RF data links to senddifferential corrections to user equipment. Alternatively, pseudolitescan be synchronized to the GPS timebase. A GPS receiver determines GPStime from a conventional code-based GPS solution using at least four GPSsatellites and passes the determined time to a co-located pseudolitetransmitter. The accuracy of the GPS timebase is constrained by GPSerror sources including ionospheric and tropospheric delay, satelliteclock error, satellite position error, receiver noise, and multipath.Time accuracies of approximately 50 to 100 nanoseconds are achievable byusing the GPS timebase method, however this translates to positionaccuracies only in the order of tens of meters. This accuracy is muchtoo coarse for precise navigation systems.

Carrier-Based Differential GPS Using an “Omni-Marker” Pseudolite

U.S. Pat. No. 5,583,513 to Cohen, titled “System and Method forGenerating Precise Code-based and Carrier Phase Position Determinations”describes a differential correction method whereby a so called“omni-marker” pseudolite serves as a channel for relaying information toa position receiver for making differential ranging corrections (Column6, lines 43 to 46). The omni-marker pseudolite can be described as ametaphorical mirror, whereby GPS satellite signals are “reflected”in-phase from the known omni-marker pseudolite position to the positionreceiver. Thus, the out-going carrier and PRN code components of each ofthe beacon marker signals is exactly phase coherent with respect totheir incoming counterparts in the GPS signals (Column 6, lines 28 to32). A position receiver situated in an over-flying aircraft receivespositioning signals from the GPS satellites and also receives“reflected” GPS positioning signals from the omni-marker pseudolite, andsubsequently computes differential range measurements.

Cohen's differential method eliminates the need for a traditionaldigital data link, as required by conventional code and carrier-baseddifferential systems. However, an omni-marker position receiver muststill receive both GPS satellites and omni-marker signals to compute adifferential range measurement. Receiving omni-marker signals alone willnot allow a position computation. Also, the omni-marker must generateand transmit individual carrier and PRN components for each GPSsatellite in view, making the omni-marker complex and expensive.Currently, this would require up to twelve individual transmissions froma single omni-marker. Further, an omni-marker position receiver requiresdouble the receive channels of a conventional differential GPS receiver,adding to the cost and complexity.

Differential Range Measurements Using “Ground Transceiver” Pseudolites

U.S. Pat. No. 6,121,928 to Sheynblat, titled “Network of GroundTransceivers” describes a differential correction method whereby anetwork of so called “ground transmitter” and “ground transceiver”pseudolites serve as channels for relaying information to a positionreceiver for the differential determination of user position (Column 5,lines 31 to 36). Sheynblat teaches the use of differential correction toovercome master clock bias (Column 5, lines 23 to 36) and line biasesintroduced by the ground transceiver hardware (Column 5, lines 38 to 67and Column 6, lines 1 to 23). Sheynblat's differential methodologies andembodiments include: (i) a user receiver differencing ground transceiversignals with a ground transmitter signal (Column 5, lines 31 to 36, andclaim 2), (ii) a user receiver differencing multiple master groundtransmitter signals with a ground transceiver (Column 6, lines 25 to 67,Column 7, lines 1 to 33), and (iii) a user receiver differencing groundtransceiver signals, which incorporate signals that have beendifferenced with a satellite signal (Column 7, lines 34 to 67, Column 8,lines 1 to 34). Sheynblat's patent teaches an advance of differentialmethodologies but does not teach, show, or suggest a highly desirablesystem that would produce single point position solutions in a rovingposition receiver from a network of ground transceivers.

Prior art systems will not allow time-of-arrival position determinationwithout requiring at least one of: (a) a physical connection betweentransmitter beacons; (b) an atomic time standard at each transmitter;(c) synchronization to a GPS timebase; or (d) some form of differentialcorrection. A system that can provide extremely precise time-of-arrivalpositioning signals, without any of these constraints, is highlydesirable. The present invention achieves this desirable goal bychronologically synchronizing a system of transceivers (hereafterreferred to as a Positioning-Unit Devices), as described below.

OBJECT OF TE INVENTION

It is an object of the present invention to provide a positioning systemand method for making precise code and carrier phase positiondeterminations without the need for physical interconnections betweenPositioning-Unit Devices.

It is yet a further object of the present invention to provide apositioning system and method for making precise code and carrier phaseposition determinations without the need of atomic time standards.

It is yet a further object of the present invention to provide apositioning system and method for making precise code and carrier phaseposition determinations without the need for a Global NavigationSatellite System timebase.

It is yet another object of the present invention to provide apositioning system and method for making precise code and carrier phaseposition determinations without the requirement of differentialcorrection techniques.

It is yet a further object of the present invention to chronologicallysynchronize Positioning-Unit Devices to a system timebase, the systemtimebase not necessarily being of absolute accuracy.

It is yet a further object of the present invention to propagate areference timebase through geographically distributed Positioning-UnitDevices.

It is yet a further object of the present invention to provide a rovingposition receiver with chronologically-synchronous code phasepseudoranges, such that single-point code phase position solutions canbe determined without the aid of differential correction.

It is yet a further object of the present invention to provide a rovingposition receiver with chronologically-synchronous carrier phasepseudoranges, such that once integer cycle ambiguities are resolved, asingle-point carrier phase position solution can be determined withoutthe aid of differential correction.

It is yet a further object of the present invention to provide a rovingposition receiver with precise network time-transfer information.

SUMMARY OF THE INVENTION

The foregoing objects of the invention are achieved by a positioningsystem that includes a Positioning-Unit Device positioned at a knownlocation with respect to a reference co-ordinate system, that receivesone or more reference positioning signals from reference transmitterspositioned at known locations with respect to a reference co-ordinatesystem. Reference transmitters include other Positioning-Unit Devices,Wide Area Augmentation System (WAAS) satellites, Global NavigationSatellite System (GNSS) satellites, Pseudolites, or any other signalsthat incorporate timing information. Each of the received referencepositioning signals preferably has a carrier component, a pseudo-randomcode component, and a data component. The Positioning-Unit Devicegenerates, in response to the received reference positioning signals andtheir known locations, a unique positioning signal. The uniquepositioning signal has a carrier component chronologically synchronizedto one or more of the carrier components of the received positioningsignals, a pseudo-random code component chronologically synchronized toone or more of the pseudo-random code components of the receivedpositioning signals, and a data component chronologically synchronizedto one or more of the data components of the received positioningsignals. Once a Positioning-Unit Device is chronologically synchronizedto a reference transmitter, other Positioning-Unit Devices entering thenetwork can use its unique transmitted positioning signal as a referencepositioning signal. The geographical distribution of chronologicallysynchronized Positioning-Unit Devices creates a time-coherent network ofpositioning signals. The method of the invention thereby allows a uniquecapacity to propagate an extremely precise timebase across a substantialgeographical area.

The system also includes at least one roving position receiver. Theroving position receiver can make code-based single point positiondeterminations by making pseudorange measurements for each of thereceived chronologically synchronized pseudorandom code components and,once carrier integer cycle ambiguity has been resolved, can makecarrier-based single point position determinations by making pseudorangemeasurements for each of the received chronologically synchronizedcarrier components. The formation of a chronologically synchronizedpositioning system allows a roving position receiver the ability toautonomously calculate both code and precise carrier-based single pointposition solutions without the requirement of differential correction.Furthermore, the requirement for absolute time accuracy within thenetwork (normally derived in prior art by atomic time standards) isnegated.

The methods described above wherein Positioning-Unit Deviceschronologically synchronize to at least one reference transmitter willhereinafter be referred to as “Time-Lock”.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of one embodiment of Time-Lockaccording to the present invention, incorporating a single referencetransmitter broadcasting to a plurality of Positioning-Unit Devices, anda roving position receiver determining an autonomous single pointposition solution.

FIG. 2 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a single referencetransmitter broadcasting to a single Positioning-Unit Device.

FIG. 3 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a single referencetransmitter broadcasting to a plurality of Positioning-Unit Devices.

FIG. 4 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a referencetransmitter broadcasting through an intermediary Positioning-UnitDevice.

FIG. 5 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a plurality ofreference transmitters broadcasting to a single Positioning-Unit Device.

FIG. 6 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a Wide AreaAugmentation System (WAAS) reference transmitter broadcasting to fourPositioning-Unit Devices. The Positioning-Unit Devices subsequentlytransmit their own unique chronologically synchronized positioningsignals to a roving position receiver situated in a satellite-occludedenvironment.

FIG. 7 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating a Positioning-UnitDevice reference transmitter broadcasting to three otherPositioning-Unit Devices. The Positioning-Unit Devices subsequentlytransmit their own unique chronologically synchronized positioningsignals to a roving position receiver.

FIG. 8 is a graphical representation of another embodiment of Time-Lockaccording to the present invention, incorporating two autonomousnetworks of Positioning-Unit Devices, and a roving position receiversituated at the boundary of the two networks. The boundaryPositioning-Unit Devices subsequently transmits inter-networkcorrections to the roving position receiver.

FIG. 9 is a block diagram of Positioning-Unit Device hardware, accordingto the present invention.

Overview

A Positioning-Unit Device is a specialized transceiver, which ispositioned at a known location and receives at least one referencepositioning signal from at least one reference transmitter. Preferably,the reference transmitter is another Positioning-Unit Device, or a WAASsatellite. In response to the received reference positioning signals,the Positioning-Unit Device chronologically synchronizes an internallygenerated positioning signal to the reference transmitter timebase andtransmits its unique positioning signal to all other position receiversin view. The minimum requirement for the formation of an autonomouslocation network is at least two Positioning-Unit Deviceschronologically synchronized to one reference transmitter. A rovingposition receiver in view of the all the transmitted signals within thisautonomous network is capable of determining autonomous code and carriersingle point position solutions without the need for differentialcorrection. Furthermore, the reference transmitter oscillator does notneed the inherent stability of atomic time standards as required byprior art systems, thereby allowing an inexpensive crystal oscillator tobe used as a reference timebase for the entire location network.

Thus, as detailed below, a Positioning-Unit Device may serve as ametaphorical “channel” for distributing chronologically synchronizedpositioning signals to a roving position receiver. This allows theroving position receiver to calculate both code and carrier-based singlepoint position determinations, without the need for physical connectionsbetween Positioning-Unit Devices, without requiring atomic timestandards or GNSS timebases, and without requiring differentialcorrection.

System and Method

FIG. 1 shows one configuration for a Positioning System that generatesprecise position determinations using code and carrier-based singlepoint position calculations. A plurality of Positioning-Unit Devices101-1 & 101-2 are positioned at known locations with respect to areference co-ordinate system and respectively receive at least onereference positioning signal 102 broadcast by at least one referencetransmitter 103, which is also positioned at a known location withrespect to a reference co-ordinate system. In response to the receivedreference positioning signal 102 the Positioning-Unit Devices 101-1 &101-2 transmit one or more unique positioning signals 104-1 & 104-2,which are chronologically synchronized to the reference transmitter 103.A roving position receiver 105, situated within the network of devices101-1, 101-2 & 10-3, receives reference positioning signals 102 from thereference transmitter 103 and unique positioning signals 104-1 & 104-2from the Positioning-Unit Devices 101-1 & 101-2, and autonomouslycalculates both code and carrier-based single point positiondeterminations from the network of chronologically synchronizedpositioning signals.

Time-Lock

Time-Locked Positioning-Unit Devices synchronize to a commonchronological timebase, which can be of arbitrary value and havearbitrary variance. Therefore any simple and inexpensive clock source,such as a crystal oscillator, will suffice as the reference clock in areference transmitter. In the preferred embodiment a temperaturecompensated crystal oscillator (TCXO) or better is used. APositioning-Unit Device first acquires a reference transmitterpositioning signal, and calculates a so-called time-of-flight offsetfrom the known co-ordinates of the reference transmitter and the knownco-ordinates of the Positioning-Unit Device. The time-of-flight offsettakes into consideration the propagation time delay experienced by thereference positioning signal whilst traveling from the referencetransmitter to the Positioning-Unit Device. In free space,electromagnetic waves travel approximately one meter every threenanoseconds. Next, the Positioning-Unit Device applies thetime-of-flight offset to an internally generated positioning signal andaligns this positioning signal to the incoming reference positioningsignal, thus bringing the internally generated positioning signal intochronological alignment with the reference transmitter. Specifically,Time-Lock is achieved when a Positioning-Unit Devices' internallygenerated positioning signal has frequency coherence with an incomingreference positioning signal, and chronological coherence with thereference transmitter timebase.

A reference positioning signal is transmitted via a radio frequency (RF)carrier from a reference transmitter. The reference positioning signalcan be generated from any valid time source, which may includePositioning-Unit Devices, Wide Area Augmentation System (WAAS)satellites, Global Navigation Satellite System (GNSS) satellites,Pseudolites, or any combination of valid sources. Referring now to FIG.2, a Positioning-Unit Device 201 located at a known distance from areference transmitter 202 receives a reference positioning signal 203transmitted by the reference transmitter 202. The reference positioningsignal 203 has a carrier component, a unique pseudo-random codecomponent, and a data component. The Positioning-Unit Device 201incorporates a position receiver 204 and a co-located transmitter 205.The position receiver 204 is capable of receiving positioning signalsfrom all reference positioning signals in view 203, and also positioningsignals from its co-located transmitter 205. In response to the receivedreference positioning signal 203, the Positioning-Unit Device 201transmits a so-called slave positioning signal 206 from its transmitter205, which is received by the Positioning-Unit Device position receiver204. The slave positioning signal 206 has a carrier component, a uniquepseudo-random code component, and a data component. The Positioning-UnitDevice position receiver 204 receives and simultaneously samples thereference positioning signal 203 from the reference transmitter 202 andthe slave positioning signal 206 from the co-located transmitter 205. Atransmission time difference is then calculated between the receivedreference positioning signal 203 and the received slave positioningsignal 206. The transmission time difference, as used in the preferredembodiment, is determined by:

-   -   (a) Comparing the integrated carrier phase (ICP) measurements        determined from the carrier components of the reference        positioning signal 203 and the slave positioning signal 206 to        determine a carrier frequency difference.    -   (b) Demodulating and comparing the navigation data components        from the reference positioning signal 203 and the slave        positioning signal 206 to determine a coarse transmission time        difference.    -   (c) Comparing the pseudorange measurements determined from the        pseudo-random code components of the reference positioning        signal 203 and the slave positioning signal 206 to determine a        code pseudorange difference.    -   (d) Comparing the instantaneous carrier phase measurements        determined from the carrier components of the reference        positioning signal 203 and the slave positioning signal 206 to        determine a carrier phase difference.

For precise time synchronization of the slave positioning signal 206 tothe reference transmitter 202 timebase the signal propagation delaybetween the reference transmitter antenna 207 and the Positioning-UnitDevice position receiver antenna 208 must be accounted for. The knowngeometrical distance in meters 209 from the reference transmitterantenna 207 to the Positioning-Unit Device position receiver antenna 208can be converted to a signal time-of-flight by the formula:time-of-flight=distance/speed of light. The Positioning-Unit Device 201incorporates a steered transmitter clock 210, which can be adjusted infrequency by the Positioning-Unit Device CPU 211. The correction to thesteered transmitter clock 210 is determined by the Positioning-UnitDevice CPU 211 from the time difference between the referencepositioning signal 203 and the slave positioning signal 206 which ismeasured by the Positioning-Unit Device receiver 204, and offset by thereference positioning signal time-of-flight 209. This brings the slavepositioning signal 206 into chronological synchronization with thereference transmitter 202 timebase.

The process of differencing the received reference positioning signal203 with the slave positioning signal 206 eliminates thePositioning-Unit Device position receiver clock term, thereby allowingthe Positioning-Unit Device 201 to follow the reference transmitter 202timebase without any clock bias caused by the local Positioning-UnitDevice oscillator 212. Furthermore, differencing between two channels ofthe same position receiver 204 eliminates any receiver line bias orgroup delay caused by the position receiver electronics.

Control States of a Positioning-Unit Device

In the preferred embodiment, Positioning-Unit Devices Time-Lock toreference transmitters using the following control states:

State 0: Reset

Reset all hardware

State 1: Acquire Reference

The Positioning-Unit Device CPU 211 initiates a search for a referencepositioning signal 203 by the Positioning-Unit Device position receiver204.

State 2: Lock to Reference

The Positioning-Unit Device position receiver 204 acquires a referencepositioning signal 203 and reference transmitter 202 position and timeis demodulated from its navigation data component by thePositioning-Unit Device CPU 211.

State 3: Synchronize Slave

The Positioning-Unit Device CPU 211 waits to allow for coarse timealignment with the reference positioning signal navigation datacomponent An internal clock generator is then initiated by the CPU 211.

State 4: Initialize Slave

The Positioning-Unit Device CPU 211 determines an appropriate and uniquePRN code sequence for this particular Positioning-Unit Device 201 andassigns this PRN code sequence to the Positioning-Unit Devicetransmitter 205. The current frequency offset for the referencepositioning signal 203 (relative to the Positioning-Unit Deviceoscillator 212) is also assigned to the Positioning-Unit Device steeredtransmitter clock 210 by the Positioning-Unit Device CPU 211. Thisserves to initialize the Positioning-Unit Device transmitter 205 to afrequency that is approximately the same as the frequency of thereference positioning signal 203. The Positioning-Unit Device CPU 211also assigns the determined PRN sequence to a free receiver channel inthe Positioning-Unit Device position receiver 204. The receiver channelis initialized with the same frequency offset and pseudorandom codephase value as the Positioning-Unit Device transmitter 205, in order toaid acquisition of the slave positioning signal 206 by thePositioning-Unit Device position receiver 204. The Positioning-UnitDevice then initiates a transmission of the slave positioning signal206.

State 5: Acquire Slave

The Positioning-Unit Device position receiver 204 initiates a search forthe slave positioning signal 206.

State 6: Lock to Slave

The Positioning-Unit Device position receiver 204 acquires the slavepositioning signal 206 and a coarse slave time is demodulated from itsnavigation data component.

State 7: Reference/Slave Frequency Lock

The simultaneous integrated carrier phase (ICP) measurements for thereference positioning signal 203 and slave positioning signals 206 areinitialized (zeroed) and differenced by the Positioning-Unit Deviceposition receiver 204. This differenced value represents the frequencyand phase difference between the reference positioning signal 203 andthe slave positioning signal 206. A control loop within the positioningunit device CPU 211, continuously applies corrections to thePositioning-Unit Device steered transmitter clock 210 to maintain a zeroICP difference between the reference positioning signal 203 and theslave positioning signal 206, thus maintaining Frequency Lock.

Alternatively the received reference positioning signal frequency offsetvalue, as measured by the Positioning-Unit Device position receiver 204,can be fed directly to the Positioning-Unit Device steered transmitterclock 210 to create a so called “Frequency Tracking System” (FTS). Thesteered transmitter clock 210 simply emulates the frequency offset ofthe incoming reference positioning signal 203, thus maintainingFrequency Lock. This method requires the Positioning-Unit Deviceoscillator 212 to be common between position receiver 204 andtransmitter 205.

State 8: Reference/Slave Code-Lock

Once State 7 Reference/Slave Frequency Lock is achieved the timedifference between the reference positioning signal 203 and the slavepositioning signal 206 can be accurately measured and any time biaseliminated. Reference/Slave Code-Lock is achieved when thePositioning-Unit Device steered transmitter clock 210 is slewed therequisite amount of time to bring the reference and slave positioningsignals into PRN code alignment. The time-of-flight value 209 is used tooffset the measured reference-slave time difference to remove the effectof the reference signal propagation delay, and the calculated timedifference is then applied as a clock correction to the Positioning-UnitDevice steered transmitter clock 210. The clock correction is achievedby engaging the Frequency Tracking System (FTS), and applying anadditional frequency offset to the steered transmitter clock 210 for apredetermined time period. This additional frequency offset allows theslave positioning signal 206 to slew in time until it becomes timecoherent with the reference transmitter 202 timebase. Once this TimeSlew is completed the control loop is re-engaged. Alternatively,Code-Lock can be achieved by slewing the Positioning-Unit Devicetransmitter 205 PRN code generator the requisite amount of code phase(chips) whilst maintaining Frequency Lock.

Code-Lock is based on PRN code accuracy, which is inherently noisy. Inthe preferred embodiment stationary Positioning-Unit Devices filter PRNcode noise to a sub carrier cycle level.

State 9: Reference/Slave Phase Lock

Once State 7 Reference/Slave Frequency Lock and State 8 Reference/SlaveCode-Lock are achieved, two time errors still remain that must becorrected: (1) a 180 degree phase ambiguity and; (2) a time-of-flightphase offset (1) Correcting a 180 degree phase ambiguity: Data isdemodulated from a PRN code positioning signal using a specializedPhase-Lock-Loop, well-known in the art as a “Costas Loop”. The CostasLoop technique inherently incorporates a 180 degree phase ambiguity, andtherefore can acquire and track positioning signals with a half cycleambiguity. This half cycle ambiguity represents an approximate 200picosecond time offset at 2.4 GHz. The Costas Loop ambiguity can beresolved by reference to a predetermined sequence of data bits,generally referred to as a preamble, transmitted in the navigation datacomponent by transmitters within the location network. When the CostasLoop ambiguity is resolved, an arbitrary fixed phase difference becomesevident between the position receiver phase registers of theFrequency-Locked reference and slave positioning signals. This arbitraryphase offset is due to the arbitrary phase of the slave positioningsignal and is adjusted in the following step (2) below.

(2) Correcting Time-of-Flight Phase Offset: A fractional-cycletime-of-flight phase offset is present due to the reference positioningsignal propagation delay between the reference transmitter antenna 207and the Positioning-Unit Device antenna 208. The geometrical distance209 between the reference transmitter and the Positioning-Unit Devicecan be represented as a number of whole carrier cycles (the integercomponent) 213, plus a fractional carrier cycle (the fractionalcomponent) 214. The time-of-flight phase offset is the fractional cycleamount 214 computed from the known geometrical distance between thereference transmitter antenna 207 and the Positioning-Unit Deviceantenna 208. The integer component 213 is corrected in the State 8Reference/Slave Code-Lock control state described above. The fractionalcomponent 214 however, is too fine to be corrected in the State 8Reference/Slave Code-Lock state, and must therefore be corrected as acarrier phase adjustment. The Frequency Tracking System (FTS) is engagedand the Positioning-Unit Device steered transmitter clock 210 is timestewed the requisite fractional-cycle amount (from its currentlymeasured arbitrary phase value determined in step (1) above) to a newlydetermined time-of-flight phase value. The Time-Lock-Loop (TLL) is thenre-engaged. The Positioning-Unit Device carrier phase slave positioningsignal 206 emanating from the Positioning-Unit Device antenna 208 is nowchronologically synchronized with the reference transmitter 202 carrierphase positioning signal emanating from the reference transmitterantenna 207.

State 10: Reference/Slave All Lock

Once all of the above states have been achieved, the CPU 211 declaresTime-Lock and the Positioning-Unit Device 201 begins transmission of itsnow fully synchronized unique positioning signal 215. ThePositioning-Unit Device unique positioning signal 215 is nowchronologically synchronized to the reference transmitter 202 timebasewith an accuracy of picoseconds, a capability that is substantiallybeyond the capacity of any prior art.

Unique Positioning Signals

In the preferred embodiment each Positioning-Unit Device transmits aunique positioning signal, which consists of a carrier component, apseudorandom code component, and a navigation data component. Thecarrier component is a sinusoidal radio frequency wave preferablytransmitted in the 2.4 GHz ISM band, though the method of the presentinvention is equally applicable to other frequency bands. Thepseudorandom number (PRN) code component is modulated upon the carriercomponent, and consists of a unique code sequence which can bedistinguished amongst other pseudorandom code sequences transmitted byother devices on the same carrier frequency. This technique is known asCode Division Multiple Access (CDMA), and is well-known in the art. Thenavigation data component is proprietary information modulated upon thepseudorandom code component, and provides a communications link totransfer navigation information to Positioning-Unit Devices and rovingposition receivers. Navigation information may include network time,Positioning-Unit Device locations, metaphorical “reference clocklineage” information, and other desired network data.

Time-Lock Configurations

Time-Lock may be implemented in many different configurations. Theseconfigurations include:

-   -   1. A single reference transmitter broadcasting to a single        Positioning-Unit Device.    -   2. A single reference transmitter broadcasting to a plurality of        Positioning-Unit Devices.    -   3. One or more reference transmitters broadcasting through        intermediary Positioning-Unit Devices    -   4. A plurality of reference transmitters broadcasting to one or        more Positioning-Unit Devices.    -   5. Point position time synchronization        A Single Reference Transmitter Broadcasting to a Single        Positioning-Unit Device.

A single reference transmitter can be used to broadcast a referencepositioning signal to a single Positioning-Unit Device. FIG. 2 shows aPositioning-Unit Device 201 situated at a known location, and areference transmitter 202 also situated at a known location. ThePositioning-Unit Device 201 receives the reference positioning signal203 transmitted by the reference transmitter 202 and the slavepositioning signal 206 transmitted by the Positioning-Unit Devicetransmitter 205. In response to the received reference positioningsignal 203 the Positioning-Unit Device 201 determines the referencepositioning signal propagation delay 209 and applies an appropriatetransmitter clock correction to chronologically synchronize the carriercomponent, unique PRN code component, and data component of itsinternally generated slave positioning signal 206 to the carriercomponent, PRN code component, and data component of the referencetransmitter positioning signal 203. The Positioning-Unit Devicesubsequently transmits a unique positioning signal 215, which ischronologically synchronized to the reference transmitter 202 timebase.

Two positioning signals are not sufficient to determine a positionsolution in a roving position receiver. However, if the referencetransmitter is a WAAS satellite the Time-Locked Positioning-Unit Devicesignal will be synchronous with GPS time to picosecond level, andtherefore can be used by a position receiver as an additional preciseranging source for a conventional code-based GPS solution.

A Single Reference Transmitter Broadcasting to a Plurality ofPositioning-Unit Devices.

A single reference transmitter can be used to form a network ofPositioning-Unit Devices when a plurality of Positioning-Unit Devices isin clear view of the reference transmitter.

FIG. 3 shows a plurality of Positioning-Unit Devices situated at knownlocations 301-1 & 301-2, and a reference transmitter 302 also situatedat a known location. The Positioning-Unit Devices 301-1 & 301-2 receivethe reference positioning signal 303 transmitted by the referencetransmitter 302. In response to the received reference positioningsignal 303 each Positioning-Unit Device 301-1 & 301-2 determines itsrespective signal propagation delay 304-1 & 304-2 from the referencetransmitter 302 and applies an appropriate transmitter clock correctionto chronologically synchronize the carrier component, unique PRN codecomponent, and data component of their internally generated positioningsignals to the carrier component, PRN code component, and data componentof the reference transmitter positioning signal 303. EachPositioning-Unit Devices subsequently transmits unique positioningsignals 305-1 & 305-2, which are chronologically synchronized to thereference transmitter 302 timebase.

One or More Reference Transmitters Broadcasting Through IntermediaryPositioning-Unit Devices.

One or more time-synchronized reference transmitters can be used to forma network of Positioning-Unit Devices, without all Positioning-UnitDevices being in clear view of a reference transmitter. In thisconfiguration the timing signal is cascaded via intermediaryPositioning-Unit Devices. When an intermediary Positioning-Unit Devicedeclares Time-Lock, subsequent Positioning-Unit Devices can use thisintermediary Positioning-Unit Device as their reference positioningsignal.

FIG. 4 shows a reference transmitter 401 situated at a known location,and a first Positioning-Unit Device 402 also situated at a knownlocation. The first Positioning-Unit Device 402 receives the positioningsignal 403 transmitted by the reference transmitter 401. In response tothe received reference positioning signal 403 the first Positioning-UnitDevice 402 determines the signal propagation delay 404 from thereference transmitter 401 and applies an appropriate clock correction tochronologically synchronize the carrier component, unique PRN codecomponent, and data component of its internally generated positioningsignal to the carrier component, PRN code component, and data componentof the reference transmitter positioning signal 403. The firstPositioning-Unit Device 402 subsequently transmits a unique positioningsignal 405, which is chronologically synchronized to the referencetransmitter 401 timebase.

A Second Positioning-Unit Device 406 situated at a known location, butnot in view of the reference positioning signals 410 due to signalobstruction caused by building 409, subsequently receives positioningsignal 405 from the first Positioning-Unit Device 402. In response tothe received positioning signal 405 the second Positioning-Unit Device406 determines the signal propagation delay 407 from the firstPositioning-Unit Device 402 and applies an appropriate clock correctionto chronologically synchronize the carrier component, unique PRN codecomponent, and data component of its internally generated positioningsignal to the carrier component, PRN code component, and data componentof the first Positioning-Unit Device positioning signal 405. The secondPositioning-Unit Device 406 subsequently transmits a unique positioningsignal 408 incorporating a carrier component, PRN code component, anddata component. This unique positioning signal 408 is chronologicallysynchronized to the first Positioning-Unit Device 402 timebase, which isalso chronologically synchronized to the reference transmitter 401timebase.

A Plurality of Reference Transmitters Broadcasting to One or MorePositioning-Unit Devices.

A plurality of time-synchronized reference transmitters can be used tobroadcast reference positioning signals to one or more Positioning-UnitDevices. In this configuration any reference signal error sources, suchas multipath and tropospheric delay, can be averaged between referencetransmitters to improve timebase accuracy.

FIG. 5 shows a Positioning-Unit Device 501 situated at a known location,and a plurality of reference transmitters 502-1 & 502-2 with commontimebase, also situated at known locations. The Positioning-Unit Device501 receives the reference positioning signal 503-1, 503-2 transmittedby the reference transmitters 502-1 & 502-2. In response to the receivedreference positioning signal 503-1, 503-2 the Positioning-Unit Device501 determines the signal propagation delays 504-1 & 504-2 from eachreference transmitter 502-1 & 502-2 and applies an appropriate clockcorrection to chronologically synchronize the carrier component, uniquePRN code component, and data component of its internally generatedpositioning signal to the carrier components, PRN code components, anddata components of the two reference transmitter positioning signals503-1 & 503-2. The Positioning-Unit Device 501 subsequently transmits aunique positioning signal 505, which is chronologically synchronized tothe timebase of the reference transmitters 502-1 & 502-2.

Point Position Time-Lock

A Positioning-Unit Device is also capable of synchronizing to a networktimebase without the geometrical distance (reference positioning signalpropagation delay) between reference transmitters and Positioning-UnitDevice being known. For this embodiment of Time-Lock, at least fourTime-Locked Positioning-Unit Devices must be in view. ThePositioning-Unit Device, requiring to enter the network, self-surveysits three-dimensional position by calculating a single point position,which incorporates the Positioning-Unit Device position receiver clockoffset The Positioning-Unit Device position receiver clock offsetaccurately provides network time (relative to the local positionreceiver clock), which the Positioning-Unit Device slave transmitter canuse as an accurate network timebase. In the preferred embodiment thePositioning-Unit Device uses a single point carrier solution todetermine accurate network time to the picosecond level, a capabilitythat is substantially beyond the capacity of prior art systems.

WAAS Reference

In the preferred embodiment a reference transmitter is a Wide AreaAugmentation System (WAAS) Satellite. WAAS satellites are geostationarycommunications satellites, which transmit GPS differential correctionsto GPS receivers. WAAS satellites also transmit a unique positioningsignal on the GPS L1 carrier frequency of 1575.42 MHz This uniquepositioning signal is accurately synchronized to GPS time, withcorrections provided for UTC. Therefore, a WAAS satellite makes an idealreference transmitter, which is synchronous to the world standardtimebase of UTC.

In the preferred embodiment a Positioning-Unit Device position receiverincorporates means for receiving positioning signals from otherPositioning-Unit Devices in the 2.4 GHz ISM band, and also positioningsignals from WAAS and GNSS satellites in the L band frequencies. APositioning-Unit Device may use a WAAS satellite as a referencetransmitter and Time-Lock its 2.4 GHz slave positioning signal to the1575.42 MHz WAAS positioning signal. Time-Lock between disparate carrierfrequencies is initiated by coherently down-converting the incoming WAASand Positioning-Unit Device carriers to a common baseband frequency inthe Positioning-Unit Device position receiver. Time-Lock is thenperformed with the methods previously described. Coherentdown-conversion requires the local oscillators in the Positioning-UnitDevice position receiver to be driven from a common oscillator. In thepreferred embodiment the common oscillator generates clock informationfor all components of a Positioning-Unit Device, including the positionreceiver, transmitter, and central processing unit. Line biases andgroup delay are taken into consideration when computing inter-frequencyTime-Lock, due to the disparate receive paths of the WAAS andPositioning-Unit Device carrier frequencies prior to down-conversion.

Referring now to FIG. 6, Positioning-Unit Devices 601-1, 601-2, 601-3 &601-4 are placed in known locations with clear view of the sky, andpreferably in elevated positions such as on top of hills 602-1 & 602-2and/or tall buildings 603-1 & 603-2. If required, a directional receiveantenna 604-1, 604-2, 604-3 & 604-4 may also be incorporated with eachPositioning-Unit Device 601-1, 601-2, 601-3 & 601-4 and directed towarda geostationary WAAS satellite 605 (though these additional antennas arepreferred but not essential for the method). Deploying directionalantennas on Positioning-Unit Devices helps to mitigate multipath andimprove received signal to noise ratios of the WAAS signal, which inturn improves reference timebase accuracy. Each Positioning-Unit Device601-1, 601-2, 601-3, & 601-4 Time-Locks to the WAAS satellite signal606, thus creating a precision UTC synchronized network with picosecondaccuracy. A position receiver 607 held by a pedestrian 608 is situatedinside a building 609. The WAAS satellite signal 606 cannot penetratethe building 609 due to its low signal power. However, Positioning-UnitDevice signals 610-1, 610-2, 610-3, & 610-4 from the Positioning-UnitDevices 601-1, 601-2, 601-3, & 601-4 can penetrate the building 609 dueto their close proximity. The position receiver 607 is capable ofreceiving Positioning-Unit Device positioning signals from all fourPositioning-Unit Devices, which allows precise single point positiondetermination in satellite occluded regions. In addition, once theposition receiver 607 has calculated a position solution, UTC can bedetermined accurately. The present invention therefore also providesprecision UTC time transfer in satellite occluded regions. Moreover,when the Position receiver 607 exits the building 609, signals from anyPositioning-Unit Devices 601-1, 601-2, 601-3 & 601-4, WAAS satellites605, or GNSS satellites in view can be used to form an overdeterminedposition solution, adding position integrity to the pedestrianscalculated position.

Intermediary WAAS Reference

Positioning-Unit Devices placed in clear view of the WAAS satellite mayalso be used as intermediary reference signals in another embodiment.Positioning-Unit Devices that are unable to receive WAAS satellitesignals may use intermediary “backbone” Positioning-Unit Devices astheir time reference source. Therefore, UTC may be distributedthroughout the network without all Positioning-Unit Devices being inclear view of the reference WAAS satellite.

Positioning-Unit Device Reference

In the event of a WAAS satellite not being available, it is preferablethat at least one Positioning-Unit Device provides the timebase for anetwork of Positioning-Unit Devices. Referring now to FIG. 7, a firstPositioning-Unit Device 701 situated at a known location is designatedas the reference transmitter and creates a system timebase from itsinternally generated clock 702. Two subsequent Positioning-Unit Devices703 & 704 situated at known locations Time-Lock to the firstPositioning-Unit Device reference positioning signal 705. A fourthPositioning-Unit Device 706, which is situated at a known location butout of range of the first Positioning-Unit Device 701, Time-Locks to thesecond Positioning-Unit Device unique positioning signal 707. Thereforethe system allows accurate cascaded time transfer through intermediaryPositioning-Unit Devices. Position receiver 708 receivestime-synchronous positioning signals 709 being transmitted by allPositioning-Unit Devices in view 701, 703, 704, & 706 and subsequentlycalculates a single point position solution. Further, the timecalculated at the position receiver 708 will bechronologically-synchronous with the reference clock 702 of thereference Positioning-Unit Device 701. The arbitrary time value of thereference clock 702 within the Positioning-Unit Device 701 is of noconsequence if the user is only concerned with position determination.If the user wishes time alignment with a global timebase, then thereference clock 702 within the reference Positioning-Unit Device 701needs to be steered to UTC.

Positioning-Unit Device Reference Steered by GNSS Timebase

In the event of a WAAS satellite signal not being available, andalignment to a global timebase is necessary for the network, it ispreferable that a reference Positioning-Unit Device be steered to UTC bya GNSS timebase. A GNSS timebase requires a position receiver,positioned at a known location, to compute a time solution using atleast one GNSS satellite. Time accuracies in the order of 50 nanosecondsare achievable using this technique. Relative time accuracy betweenPositioning-Unit Devices, which are Time-Locked to the referencePositioning-Unit Device, will remain at the picosecond level.

Inter-Network Position Solutions

A plurality of reference transmitters may be used to create a pluralityof autonomous networks. An autonomous network has its own uniquetimebase, which is generated by the reference transmitter. Positionreceivers that are situated within a single autonomous network candetermine position, velocity, and time (PVT) using a single pointposition solution. The position receiver's time will be determinedrelative to the network timebase (i.e. the reference transmitter clock)and is termed an intra-network position solution.

Position receivers that are located at the boundary of two autonomousnetworks, and receiving positioning signals from Positioning-UnitDevices from both networks, must first distinguish between the twonetwork timebases before determining their position. This can bedescribed as an inter-network position solution, and requires a rovingposition receiver to first chose a single timebase and apply clockcorrections to the second timebase before computing a single pointposition solution.

In the preferred embodiment, Positioning-Unit Devices also includenetwork identification Network I.D.) information in their network data.Network I.D. maps the reference-time interconnectivity ofPositioning-Unit Devices, such that Positioning-Unit Devices andposition receivers can determine the origin and metaphorical “lineage”of reference clock data for each Positioning-Unit Device in view. Thisallows a Positioning-Unit Device or position receiver located at theboundary of two autonomous networks to determine which Positioning-UnitDevices are associated with each network, and therefore whichPositioning-Unit Devices require clock correction within the rovingposition receiver position calculations.

Each Positioning-Unit Device receives Network I.D. information from allother Positioning-Unit Devices in view, and in response generates andtransmits its own Network I.D. information to all other Positioning-UnitDevices and roving position receivers in view.

Referring now to FIG. 8, there is depicted two autonomous networks ofPositioning-Unit Devices 801 & 802.

Positioning-Unit devices 801-1, 801-2, and 801-3 are in view of oneanother and communicate to each other via positioning signals 803-1,803-2, and 803-3. Positioning-Unit devices 802-1, 802-2, and 802-3 arein view of one another and communicate to each other via positioningsignals 804-1, 804-2, and 804-3. A Positioning-Unit Device situated nearthe boundary of the two networks 801-3 receives Positioning-Unit Devicepositioning signals 804-3 from an adjacent-network Positioning-UnitDevice 802-3 and measures the timebase difference, or clock bias, of theadjacent network timebase with respect to its own network 801 timebase.The Positioning-Unit Device 801-3 transmits clock corrections for theadjacent-network Positioning-Unit Devices 802-1, 802-2, & 802-3 in itsnetwork data, which is incorporated in its positioning signal 803-3.Positioning signals from only one adjacent-network Positioning-UnitDevice 802-3 needs to be received by Positioning-Unit Device 801-3 whenforming a network correction value, as all clocks in an autonomousnetwork are time coherent. Furthermore, only one Positioning-Unit Device801-3 need measure an adjacent network, as its transmitted network clockcorrections which are sent in the network data of its positioning signal803-3, are received and relayed to other Positioning-Unit Devices withinits own network 801, for subsequent transmission 803-1 & 803-2 to rovingposition receivers 805.

The transmitted correction value, transmitted in the network data of thePositioning-Unit Device 801-3 positioning signal 803-3, is received by aposition receiver 805 that is roving between networks 801 & 802. Theroving position receiver applies the received network clock correctionsfrom Positioning-Unit Device 801-3 and subsequently calculates a singlepoint position solution using all Positioning-Unit Device positioningsignals in view 803-1, 803-2, 803-3, and adjacent networkPositioning-Unit Device positioning signal 804-3. With a single pointposition solution calculated the roving position receiver 805 clock willbe time coherent with the network 801 timebase that provided the clockcorrections. Furthermore, the adjacent network Positioning-Unit Device802-3 can also receive positioning signals 803-3 from the firstPositioning-Unit Device 801-3 and measure the timebase difference of thefirst network 801 with respect to its own network 802 timebase. Theadjacent-network Positioning-Unit Device 802-3 then transmits clockcorrections for the its adjacent-network Positioning-Unit Devices 801-1,801-2, & 801-3 in its network data within its positioning signal 804-3,thereby allowing roving position receivers 805 to choose betweentimebases, if required.

Multiple Frequency Time-Lock

In the preferred embodiment a plurality of positioning signals aretransmitted on a plurality of frequencies from each Positioning-UnitDevice. Position receivers subsequently interpret the plurality ofpositioning signals to generate a so called wide-lane for integercarrier cycle ambiguity resolution (AR). RF carrier signals experience atime delay whilst passing through transmitter and receiver electronics,known as “group delay”. Group delay can vary many nanoseconds, dependingon frequency and ambient temperature. Therefore, a plurality of carrierfrequencies generated from a common oscillator and transmitted throughthe same transmit path will experience unequal time delays due to thecarrier frequency differences, and further experience varying timedelays caused by temperature change of transmitter electronics. Thiscauses transmitted positioning signals that are not phase coherent. Nonphase-coherent positioning signals will induce range errors into thewide-lane ambiguity resolution (AR) process.

A Positioning-Unit Device can eliminate the non-coherent phase problemfrom a reference transmitter by transmitting a plurality offrequency-diverse positioning signals, which are individuallytime-locked to their respective incoming reference positioning signals.A Positioning-Unit Device incorporates a plurality of steeredtransmitter clocks, capable of steering a plurality of positioningsignals, which are transmitted on a plurality of carrier frequencies.The Positioning-Unit Device position receiver tracks the plurality offrequency-diverse reference positioning signals, and also tracks theplurality of frequency-diverse slave positioning signals. ThePositioning-Unit Device Time-Locks each frequency-diverse referencepositioning signal to its respective frequency-diverse slave positioningsignal, such that each slave positioning signal is chronologicallysynchronized with the reference transmitter. The Positioning-Unit Devicethen transmits its plurality of frequency-diverse positioning signals,which are time-coherent with the group delay from the referencetransmitter. With at least three time-locked Positioning-Unit Devices inview, a position receiver determines wide-lane integer ambiguityresolution (AR) from each Positioning-Unit Device in view. The referencetransmitter group delay has created an AR range error, which is commonamongst the Time-Locked Positioning-Unit Devices. Therefore the same ARinduced range error is evident on each Positioning-Unit Devicepseudorange. The position receiver interprets this common pseudorangeerror as a receiver clock bias and eliminates the error in the singlepoint position calculation.

Network Co-Ordinate Frame

A prerequisite for Time-Lock is the knowledge of the Positioning-UnitDevice positions with respect to a reference co-ordinate frame. Anyvalid co-ordinate frame may be used, but in the preferred embodiment theEarth Centered Earth Fixed (ECEF) co-ordinate frame is used, which isalso the co-ordinate frame used by GPS and WAAS. In the preferredembodiment, Positioning-Unit Devices self-survey from GNSS, and/or WAAS,and/or other Positioning-Unit Devices to determine an ECEF co-ordinate.

Transmission Frequency

In the preferred embodiment, Positioning-Unit Devices transmit in theunlicensed Industrial Scientific Medical (ISM) band of 2.4 GHz to 2.48GHz. The 2.4 GHz ISM band allows the development of Positioning-UnitDevice networks without regulatory constraint, and without interferenceto current navigation systems such as GPS. The 2.4 GHz ISM band alsoallows 83.5 MHz bandwidth, which can be used for increased chippingrates of direct sequence spread spectrum positioning signals, or the useof multiple carriers for widelane integer cycle ambiguity resolution.

Description of Positioning-Unit Device Hardware

In the preferred embodiment, a Positioning-Unit Device incorporates aposition receiver, a transmitter, a central processing unit (CPU), and acommon oscillator. The position receiver incorporates a plurality ofreceive channels capable of receiving a plurality of positioningsignals, each comprising a carrier component, a PRN code component, anda data component. The transmitter incorporates at least one RF carriergenerator, at least one PRN code generator, and at least one steeredclock. The CPU comprises means for interpreting positioning signalsreceived by the position receiver, responsive means to control thetransmitter steered clock and means to generate navigation data. Thecommon oscillator provides a coherent local timebase for all componentsof the Positioning-Unit Device.

Referring now to FIG. 9, there is depicted a Positioning-Unit Device 901incorporating a position receiver 902, a transmitter 903, a CentralProcessing Unit (CPU) 904, and a common oscillator 905. The positionreceiver 902 incorporates a plurality of receive channels 906, and thetransmitter 903 incorporates one or more of carrier generator 907, oneor more of code generator 908, and one or more of steered clock 909. TheCPU 904 includes means for position receiver communication 910, meansfor transmitter communication 911, and means for transmitter steeredclock communication 912.

Positioning-Unit Device Position Receiver

A Positioning-Unit Device position receiver comprises at least onereceive channel capable of receiving and demodulating at least onereference positioning signal from a reference transmitter, and at leastone receive channel capable of receiving and demodulating at least oneco-located transmitter slave positioning signal. Preferably, aPositioning-Unit Device position receiver is capable of receiving aplurality of reference positioning signals for increased accuracy andintegrity. The Positioning-Unit Device position receiver preferablyshould also be capable of receiving positioning signals from otherPositioning-Unit Devices transmitting in the 2.4 GHz ISM band, andpositioning signals from WAAS and GNSS satellites transmitting in themicrowave L band frequencies. A Positioning-Unit Device positionreceiver tracks, demodulates, and interprets positioning signalsutilizing the same methodologies used in conventional GPS receiverdesign. GPS receiver processing and design are well-known in the art andare not a subject described here.

Positioning-Unit Device Transmitter

A Positioning-Unit Device transmitter has many similarities to aconventional GPS pseudolite, with one major and critical improvement: asteered transmitter clock. In the preferred embodiment the steeredtransmitter clock is generated in the digital domain using DirectDigital Synthesis (DDS) techniques. DDS technology produces a digitallygenerated oscillator, which can be frequency controlled to millihertzaccuracies, thus allowing the transmitter clock to be precisely “slaved”to an incoming reference signal. The transmitter also incorporates atleast one radio frequency (RF) carrier generator, and at least onepseudorandom number (PRN) code generator. The RF carrier generatorproduces the carrier component, which is a sinusoidal radio frequencywave, preferably transmitted in the 2.4 GHz ISM band, and the PRN codegenerator produces the code component, which comprises a unique codesequence that can be distinguished amongst other pseudorandom codesequences transmitted on the same carrier frequency. A plurality ofcodes can be generated on a plurality of frequencies to produce a socalled “wide lane”, which allows carrier integer cycle ambiguity to beresolved in a roving position receiver. In the preferred embodimentPositioning-Unit Device transmitters are pulsed in a Time DivisionMultiple Access (TDMA) scheme, such that high power CDMA positioningsignals do not jam weaker CDMA positioning signals transmitted on thesame carrier frequency. This phenomenon is known as the “near/farproblem” and is also well-known in the art.

Positioning-Unit Device Central Processing Unit

The Positioning-Unit Device CPU comprises:

a) Means to determine the current position of the Positioning-UnitDevice.

Position determination can be achieved through either self-survey orthrough manual initialization.

Self-survey requires the Positioning-Unit Device to be in view of atleast four other reference Positioning-Unit Devices to determine athree-dimensional single point position solution, or alternatively, aPositioning-Unit Device may be in view of at least three GNSS satellitesplus at least one reference Positioning-Unit Device. In this embodimentthe reference Positioning-Unit Device supplies both code and carrierdifferential corrections for all GNSS satellites in view to thePositioning-Unit Device. The Positioning-Unit Device then calculates anaccurate position relative to the reference Positioning-Unit Device.

Manual initialization is achieved by placing the Positioning-Unit Deviceat a predetermined location and manually entering the geographicalcoordinate values into Positioning-Unit Device memory. In the preferredembodiment a first Positioning-Unit Device is manually initialized usingprecisely known coordinates, with subsequent Positioning-Unit Devicesself-surveying from GNSS satellites and the first Positioning-UnitDevice.

b) Means to initiate a reference signal search by the position receiver.

All channels of the position receiver are set to search for anyreference positioning signal in view.

c) Means to acquire at least one reference positioning signal andextract network time and network data from the navigation datacomponent.

d) Means to determine the signal propagation delay from the referencetransmitter to the Positioning-Unit Device.

Reference transmitter position coordinates are first extracted from thereference positioning signal navigation data, and compared to the knownPositioning-Unit Device location. The computed geometrical distancebetween reference transmitter and Positioning-Unit Device is convertedinto a time-of-flight offset.

e) Means to initialize the slave transmitter code generator with anappropriate unique PRN code.

f) Means to generate and pass appropriate network time and network datato the transmitter, which is transmitted as the navigation datacomponent in the slave positioning signal.

Navigation Data is modulated upon the transmitter-generated PRN code,which is subsequently modulated upon the transmitter-generated RFcarrier. Navigation data includes time-of-week information,Positioning-Unit Device location, and other network data such aslocation and status of other Positioning-Unit Devices and GNSSsatellites.

g) Means to apply the calculated time-of-flight offset and initializethe slave transmitter to approximate network time and frequency.

h) Means to initiate the position receiver to search for the slavepositioning signal.

i) Means to acquire the slave positioning signal and apply a controlloop to obtain frequency coherence between the reference and slavepositioning signals.

The CPU measures the instantaneous integrated carrier phase (ICP)difference of the reference and slave positioning signals and applies acontrol loop, known as a “Time-Lock-Loop (TLL)”. The output of the TLLapplies correction values to the steered transmitter clock, in order tozero the ICP difference.

j) Means to extract the transmitted slave time from the slavepositioning signal navigation data component and determine the timedifference between the reference positioning signal and slavepositioning signal.

k) Means to Time Slew the steered transmitter clock the requisite amountto zero the time difference between the reference positioning signal andthe slave positioning signal, such that the slave positioning signal ischronologically aligned with the reference transmitter time.

l) Means to declare Time-Lock status.

Common Oscillator

The common oscillator provides a coherent local timebase for allcomponents of the Positioning-Unit Device. In particular, the sameoscillator is used to drive the position receiver, the CPU, and thesteered transmitter clock. A coherent local timebase allows open-loopfrequency tracking of the received reference positioning signal using aso called Frequency Tracking System (FTS). With FTS the receivedreference positioning signal frequency offset, as measured by thePositioning-Unit Device position receiver, is fed directly to thePositioning-Unit Device steered transmitter clock. The steeredtransmitter clock simply emulates the frequency offset value of theincoming reference positioning signal, thus eliminating the commonoscillator term and maintaining Reference/Slave Frequency Lock betweenthe reference and slave positioning signals. FTS aids in the acquisitionand time adjustment of the slave positioning signal.

Description of the Mobile System

A roving position receiver preferably comprises a plurality of receivechannels that are capable of receiving and interpreting positioningsignals from Positioning-Unit Devices, which are preferably transmittingin the 2.4 GHz ISM band. The roving position receiver is also preferablycapable of receiving and interpreting positioning signals from GNSS andWAAS satellites transmitting in the L band frequencies. The rovingposition receiver is preferably capable of demodulating navigation dataincorporating network data from all positioning signals in view. Thisallows determination of Positioning-Unit Device network time, GNSS time,Positioning-Unit Device locations, satellite locations, and othernetwork and GNSS data. In the preferred embodiment network time isderived from GNSS time via WAAS satellites, thereby making network timeand GNSS time time-coherent. A roving position receiver also preferablyincorporates means to make code-based pseudorange measurements for eachpositioning signal in view, means to make carrier phase measurements foreach positioning signal in view, and means to solve for position,velocity, and time (PVT) using single point position determination.Single point position determination can be accomplished by using aconventional GPS position solution, which is generally a form of leastsquares regression that is well known in the art.

The roving position receiver preferably incorporates means to determineinteger cycle ambiguity. In the preferred embodiment integer cycleambiguity is resolved using wide-lane techniques. Once integer cycleambiguity is resolved, a precise carrier phase pseudorange is determinedfrom the roving position receiver to the Positioning-Unit Device. Thecarrier pseudorange comprises an integer number of carrier cycles (theinteger component) plus a fractional carrier cycle amount (fractionalcomponent or phase component), and is termed a pseudorange due to theunknown position receiver clock bias. Time-Locked Positioning-UnitDevices exhibit time coherency to tens of picoseconds, thereby allowinga single point position solution to be formed from the precise carrierpseudoranges without the need for differential correction.

A position receiver tracks, demodulates, and interprets positioningsignals generated by a network of Time-Locked Positioning-Unit Devicesutilizing the same methodologies used in conventional GPS receiverdesign. GPS receiver processing and design, as well as Wide-LaneAmbiguity Resolution, are well-known in the art and are not subjectsdescribed here.

It will of course be realized that whilst the above has been given byway of an illustrative example of this invention, all such and othermodifications and variations hereto, as would be apparent to personsskilled in the art, are deemed to fall within the broad scope and ambitof this invention as is herein set forth.

1. A method of chronologically synchronizing a unique positioning signalgenerated by a positioning-unit device at a known location with at leastone reference transmitter generating a reference positioning signal at aknown location, the method comprising said positioning-unit device: a)receiving and interpreting said reference positioning signal; b)obtaining a reference positioning signal propagation delay between saidreference transmitter and said positioning-unit device; c) generatingand transmitting a unique positioning signal; d) receiving andinterpreting the unique positioning signal transmitted in step (c); e)comparing said received and interpreted reference positioning signal andsaid received and interpreted unique positioning signal of step (d) todeduce a transmission difference; and f) adjusting continuously thegeneration of said unique positioning signal of step (c) by i) saiddeduced transmission difference; and ii) said reference positioningsignal propagation delay such that said unique positioning signal ischronologically synchronized to said reference transmitter timebase. 2.The method of claim 1, wherein said adjusted and generated uniquepositioning signal of step (f) functions as a reference positioningsignal for other positioning-unit devices.
 3. The method of claim 1,wherein said deduced transmission difference includes comparison ofintegrated carrier phase measurements determined from carrier componentsof said reference positioning signal and said unique positioning signalto determine a carrier frequency difference.
 4. The method of claim 1,wherein said deduced transmission difference includes comparison ofnavigation data components from said reference positioning signal andsaid unique positioning signal to determine a transmission timedifference.
 5. The method of claim 1, wherein said deduced transmissiondifference includes comparison of pseudorange measurements determinedfrom pseudorandom code components of said reference positioning signaland said unique positioning signal to determine a pseudorandom codepseudorange difference.
 6. The method of claim 1, wherein saidcomparison of step (e) includes comparison of instantaneous carrierphase measurements determined from carrier components of said referencepositioning signal and said unique positioning signal to determine aninstantaneous carrier phase difference.
 7. The method of claim 1,wherein said at least one reference transmitter includespositioning-unit devices, Wide Area Augmentation System satellites,Global Navigation Satellite System satellites, Pseudolites, or any othersignals that incorporate timing information.
 8. The method of claim 1,wherein said signal propagation delay of step (b) is obtained byinterpreting said reference transmitter's known location contained insaid reference positioning signal, and said positioning-unit device'sknown location contained in said unique positioning signal.
 9. Apositioning-unit device for chronologically synchronizing a uniquepositioning signal generated at a known location with at least onereference transmitter generating a reference positioning signal at aknown location, the device comprising: a) means for receiving andinterpreting said reference positioning signal; b) means to obtain areference positioning signal propagation delay between said referencetransmitter and said positioning-unit device; c) means for generatingand transmitting said unique positioning signal; d) means for receivingand interpreting said unique positioning signal transmitted in step (c);e) means for comparing said received reference positioning signal andsaid received unique positioning signal of step (d) to deduce atransmission difference; and f) means for continuously adjusting thegeneration of said unique positioning signal of step (c) by i) saiddeduced transmission difference; and ii) said reference positioningsignal propagation delay such that said unique positioning signal ischronologically synchronized to said reference transmitter timebase. 10.The device of claim 9, wherein said receiving and interpreting means of(a) above are further configured to receive unique positioning signalstransmitted by other positioning unit devices and using said receivedunique positioning signals transmitted by other positioning unit devicesas said reference positioning signal.
 11. The device of claim 9, whereinsaid means for comparing includes means for comparison of integratedcarrier phase measurements determined from carrier components of saidreference positioning signal and said unique positioning signal todetermine a carrier frequency difference.
 12. The device of claim 9,wherein said means for comparing includes means for comparison ofnavigation data components from said reference positioning signal andsaid unique positioning signal to determine a transmission timedifference.
 13. The device of claim 9, wherein said means for comparingincludes means for comparison of pseudorange measurements determinedfrom pseudo-random code components of said reference positioning signaland said unique positioning signal to determine a pseudo-random codepseudorange difference.
 14. The device of claim 9, wherein means forcomparing includes means for comparison of instantaneous carrier phasemeasurements determined from carrier components of said referencepositioning signal and said unique positioning signal to determine aninstantaneous carrier phase difference.
 15. The device of claim 9,wherein said at least one reference transmitter includespositioning-unit devices, Wide Area Augmentation System satellites,Global Navigation Satellite System satellites, Pseudolites, or any othersignals that incorporate timing information.
 16. The device of claim 9,wherein means for obtaining includes means to identify said referencetransmitter's known location contained in said reference positioningsignal, and means to identify said positioning-unit device's knownlocation contained in said unique positioning signal, to calculate saidsignal propagation delay of step (b).
 17. A system configured todetermine the position of a roving position receiver, comprising: a) atleast one reference transmitter at a known location configured togenerate and transmit reference positioning signals; b) positioning-unitdevices at known locations, each Positioning-unit device comprising: i)means for receiving signals transmitted by both or either of said atleast one reference transmitter and said positioning-unit devices; ii)means for interpreting said received signals and in response generatinga unique positioning signal that is synchronized with said referencetransmitter; and iii) means for transmitting said unique positioningsignal; c) said roving position receiver configured to receive saidunique positioning signals, and subsequently calculate a single-pointposition solution.
 18. The system of claim 17, wherein said uniquepositioning signals have frequency coherence with said received signals,and chronological coherence with said reference transmitter's timebase.19. The system of claim 17, wherein said at least one referencetransmitter includes one or more of positioning-unit devices, Wide AreaAugmentation System satellites, Global Navigation Satellite Systemsatellites, Pseudolites, or any other signals that incorporate timinginformation.
 20. The system of claim 17, wherein said unique positioningsignals include a carrier component, a pseudorandom code component and adata component, and wherein said roving position receiver determinespseudorandom code single point position determinations for each of thereceived pseudorandom code components and data components.
 21. Thesystem of claim 17, wherein said unique positioning signals include acarrier component, a pseudorandom code component and a data component,and wherein said roving position receiver determines carrier singlepoint position determinations for each of the received carriercomponents.
 22. For location, a method of generating frequency coherencebetween a received reference positioning signal and a generated andtransmitted unique positioning signal, the method comprising: a)deploying a positioning-unit device comprising: i) means to receive saidreference positioning signal; ii) means to generate said uniquepositioning signal; iii) means to adjust said unique positioning signalwith a frequency steerable clock; iv) means to supply a commonoscillator signal to said receive means and said frequency steerableclock; and v) means to transmit said unique positioning signal; b)receiving said reference positioning signal and measuring its frequencyoffset relative to said common oscillator; c) generating said uniquepositioning signal; d) adjusting said unique positioning signal byapplying said measured frequency offset to said frequency steerableclock, said steerable clocked referenced to said common oscillator; e)transmitting said unique positioning signal, such that the frequency ofthe transmitted unique positioning signal is aligned with the frequencyof the reference positioning signal.
 23. A method as claimed in claim22, wherein said frequency steerable clock is adjusted by an additionalfrequency offset for a predetermined time period to chronologically slewsaid unique positioning signal.
 24. A positioning-unit device forgenerating frequency coherence between a received reference positioningsignal and a generated and transmitted unique positioning signal, thedevice comprising: a) means configured to receive said referencepositioning signal and measure its frequency offset relative to anoscillator; b) means configured to generate said unique positioningsignal; c) means configured to adjust said unique positioning signalwith a frequency steerable clock, referenced to said oscillator, andresponsive to said frequency offset; d) means to transmit said uniquepositioning signal.
 25. A device as claimed in claim 24, wherein saidfrequency steerable clock is further configured to be responsive to anadditional frequency offset applied for a predetermined time period tochronologically slew said unique positioning signal.
 26. In a rovingposition receiver, a method for determining and correcting for adifference in the timebase of adjacent autonomous positioning networks,each network comprising a plurality of positioning unit devices in knownpositions synchronized to a local reference timebase, the methodcomprising: a) positioning unit devices of a first autonomouspositioning network receive signals from positioning unit devices of anadjacent second autonomous positioning network, measure the timebasedifference with respect to the first autonomous positioning network'stimebase, and in response calculate a clock correction; b) positioningunit devices from said first autonomous positioning network subsequentlytransmit said calculated clock correction within the network dataportion of their transmitted first autonomous positioning networksunique positioning signals; c) a roving position receiver that islocated in an area wherein it receives signals from positioning unitdevices within both networks applies said calculated clock correctionsto said signals from positioning unit devices of an adjacent secondautonomous positioning network before calculating a single pointposition solution, such that signals from networks synchronized todifferent local reference timebases can be used for a position solution.27. For location, a system for creating single point wide-lane ambiguityresolution solutions, the system comprising: a) at least one referencetransmitter at a known location configured to generate and transmit aplurality of positioning signals at a plurality of frequencies; b)positioning unit devices at known locations, each positioning unitdevice comprising: i) means for receiving signals transmitted by both oreither of said at least one reference transmitter and said positioningunit devices; ii) means for interpreting said received signals and inresponse generating corresponding unique positioning signals on aplurality of frequencies which are synchronized to said receivedsignals; and iii) means for transmitting said generated correspondingunique positioning signals; c) a roving position receiver configured toresolve wide-lane carrier phase ambiguity and subsequently calculate asingle point position solution.
 28. A system as in claim 27, whereinsaid at least one reference transmitter at a known location isconfigured to generate and transmit a plurality of non-coherentpositioning signals at a plurality of frequencies.
 29. A system as inclaim 27, wherein further means are configured to calculate apropagation delay offset between said at least one reference transmitterand said positioning unit devices and subsequently adjust said generatedcorresponding unique positioning signals by said calculated propagationdelay offset.